Mortality from dementia in occupations at risk of exposure to bovine spongiform encephalopathy: analysis of death registrations   Commentary: Uncertainty over length of incubation tempers optimism

Abstract

Since the early 1980s close contact with animals or animal products infected with bovine spongiform encephalopathy has posed a putative risk of infection with Creutzfeldt-Jakob disease. Several groups with potentially high exposure have already been identified. 1To study whether transmissible spongiform encephalopathy has had any effect on people working in animal husbandry and slaughter, we used national mortality records to examine patterns of mortality from Creutzfeldt-Jakob disease and other dementias during 1979-80 and 1982-96. Subjects, methods, and resultsWe studied people who died aged 20-74 years during 1979-1996 in England and Wales and for whom the occupational information recorded at death included butcher and abattoir worker, farmer and farm worker, or veterinarian. Women were selected on the basis of their own occupation, if recorded, or on the occupation of their spouse.The causes of death selected for study were Creutzfeldt-Jakob disease (ICD-9 046.1) and a wide range of dementias, including those most likely to be misdiagnosed as Creutzfeldt-Jakob disease.2 It was not possible to separate deaths from new variant disease. There were in any case too few deaths from new variant Creutzfeldt-Jakob disease within the study period (13 deaths during 1994-6 3 ) to include them as a separate category.We calculated the age standardised proportional mortality ratio for each occupational group and each disease category. The 95% confidence intervals were calculated by assuming that the observed number of events followed a Poisson distribution. The data were grouped into four periods (1979-80 and 1982-3, 1984-7, 1988-91, and 1992-6). Sex specific linear trends in the proportional mortality ratio over time were examined by fitting a Poisson regression model 4 to the observed numbers of deaths in each year, with the year and the expected number of deaths as the explanatory variables. The estimated risk ratios for successive calendar years represent the average linear change over time in the proportional mortality ratios.During the study period, 4 145 898 deaths were registered in people aged 20-74. Of these, 92 365 occurred in farmers, 22 596 in butchers, and 970 in veterinarians. In farmers and farm workers only 12 deaths were attributed to Creutzfeldt-Jakob disease (table). Deaths from dementia (including CreutzfeldtJakob disease seemed to fall in men but there were no significant trends. In women the proportional mortality ratios remained relatively constant. No deaths were recorded from Creutzfeldt-Jakob disease in butchers or abattoir workers. In men, the proportional mortality ratio for dementias increased from 62 (95% confidence interval 23 to 134) in 1979-83 to 119 (70 to 188) in 1992-6, but the change was not significant. In women, there was no evidence of a linear trend. No Numbers of deaths, proportional mortality ratios (PMR), and 95% confidence intervals for Creutzfeldt-Jakob disease (CJD) and dementia among men and women aged 20-74 in selected occupational groups, England and Wales, 1979-96